1. Field of the Invention
The present invention relates to a novel method of bonding a cation exchange membrane of a fluorinated polymer. More particularly, it relates to a novel method of bonding a cation exchange membrane of a fluorinated polymer for an electrolysis which is used for an electrolysis of an aqueous solution of an alkali metal chloride to produce chlorine and an alkali metal hydroxide.
2. Description of the Prior Arts
It has been known to produce chlorine in an anode compartment and alkali metal hydroxide in a cathode compartment by partitioning the anode compartment and the cathode compartment with a diaphragm and feeding an aqueous solution of an alkali metal chloride into the anode compartment to electrolyze it. The method has been known as a two-compartment type diaphragm electrolysis.
Thus, it has been proposed to use a cation exchange membrane of fluorinated polymer which does not substantially pass the electrolyte and selectively pass only alkali metal ions and has high alkali resistance and chlorine resistance, to produce an alkali metal hydroxide which does not substantially contain an alkali metal chloride as the ion exchange membrane electrolysis.
It has been found that the cation exchange membranes of the carboxylic acid type fluorinated polymer are superior to cation exchange membranes of the sulfonic acid type fluorinated polymer in the electrolytic characteristics.
Thus, an alkali metal hydroxide having a high concentration such as 25 to 45 wt. % can be produced at higher than 90% of a current efficiency by using the cation exchange membrane of the carboxylic acid type fluorinated polymer, and the membrane has rather less electric resistance in comparison with the cation exchange membrane of the sulfonic acid type fluorinated polymer.
It is often necessary, for some applications, to bond such cation exchange membranes either to themselves or to other resins.
For example, it is necessary to bond the cation exchange membrane for repairing the membrane when pin-holes, peelings or breaking at a part of the membrane is caused. It is also necessary to bond the membranes to form a specific shape such as a bag shape by bonding two or more membranes or to fix the membrane on the other synthetic resin in one piece.
In the bonding operation, it is necessary to melt the membrane enough to give a viscosity required for bonding the membrane at the bonding part, except using adhesives.
The inventors have studied to bond the cation exchange membrane of the carboxylic acid type fluorinated polymer and have found the following fact. The melting properties of the membrane are significantly different depending upon the kind of the carboxylic acid type ion exchange groups of the membrane.
It is known that membranes containing carboxylic ion exchange groups, such as the membranes employed in the electrolysis of aqueous alkali metal halides, readily decompose at low temperatures, i.e., either at or below their melting point. Thus, it has not been possible to melt bond such membranes.
When the ion exchange groups of the membrane are in the form of --(COO).sub.m X wherein X represents an alkali metal or an alkaline earth metal or HNR.sub.1 R.sub.2 R.sub.3 and R.sub.1 to R'.sub.3 represent hydrogen or a lower alkyl group and m is a valence of X, the membrane is decomposed before or during melting it whereby it is impossible to bond the membrane.